Hard to see how that translation could have come about, since there is no reference to a leopard, or any animal, in the Chinese. The leopard can only be in there by reference to translations of the entire idiom as "a leopard can't change its spots", but then I'd expect google translate to just give you "a leopard can't change its spots".

The "mettle difficult to move" bit is a fairly direct translation of the last four characters. CC-CEDICT glosses this saying as "rivers and mountains are easy to change, man's character much harder", which again renders the characters fairly directly.